The invention relates to the field of accelerometers and more particularly, to servoed linear accelerometers utilizing capacitance pick-off.
With the increasing use of accelerometers in small aircraft and missile navigation systems, as well as other applications requiring lightweight electronic systems, the reduction in size and weight in accelerometers has become increasingly important. In addition, accelerometers are frequently used in applications where they are subject to relatively severe physical shocks, vibration and radical temperature changes, all of which can affect the accuracy of the device. Thus, it is considered important to reduce the number of parts as well as reduce the weight and size of the assembly so that the effects of shock and temperature can be minimized.
Prior art servoed accelerometers also generally utilize a servo feedback circuit including a load resistance or other means to measure a current through a position restoring coil wherein the current is utilized as a measure of acceleration. However, the impedance of the load resistor can affect the servo loop gain of the feedback circuit and a change in the value of the load resistor can produce an unstable feedback loop or an undesirable frequency response of the system. Many of the prior art systems also measured the voltage across a position restoring coil instead of current which can lead to significant calibration errors due to a number of factors including the variability with temperature and current of the impedance of the force restoring coil. Another source of error or calibration difficulties with capacitive pick-off systems results from the common use of a fixed capacitor for comparison with a second variable capacitor comprised of an acceleration responsive pendulum and a capacitor plate wherein differences in capacitance between the fixed capacitor and the variable capacitor are used as a measure of acceleration. The use of such a fixed capacitor is subject to errors from stray capacitances from the accelerometer frame and housing, thus providing significant calibration difficulties. In addition, the prior art accelerometers that utilize capacitive pick-off techniques are structured with relatively large housings due to the relatively large capacitor plates used for measuring the deflection of the pendulum or proof mass. Along with the relatively large housings and associated mechanical components required by relatively large capacitors, prior art accelerometers typically require an expanded housing for containing associated electronics, or alternatively utilize a separate package for the associated electronics. Since prior art accelerometer systems are normally comprised of a force sensor and separate electronic circuits, they require the increased labor and installation costs associated with multipart assemblies along with additional electrical connections that provide potential sources of failure especially in hostile environments.